A key plot device of avengers: Endgame is
Time travel. A central point is that they have to be very careful with time travel, as explained by some team members when Rhodes asked why can't they simply go back in time and kill baby Thanos. Someone mentions that "all the rules apply, no interaction with our past selves, no trying to change the future, etc...". When Hulk meets The Ancient One, she explains that timelines and realities can be damaged, etc.
The only thing they can really pull off without messing up history is retrieving the Infinity Stones, using them in the present, and then returning them to the proper place in history to avoid any disruptions to the time line.
But then in the final scene, we discover that Captain America
went back in time and had the life that he always wanted to have with Agent Carter.
Doesn't this contradict the above mentioned dangers
of time travel? Wouldn't his getting together with Agent Carter at the very least completely alter the history of the S.S.R and S.H.I.E.L.D (if not the entire U.S military) and hence completely mess up the time line?